3 # ABSTRACT: Minimal try/catch with proper preservation of $@
10 use Exporter 5.57 'import';
11 our @EXPORT = our @EXPORT_OK = qw(try catch finally);
14 $Carp::Internal{+__PACKAGE__}++;
17 my $su = $INC{'Sub/Util.pm'} && defined &Sub::Util::set_subname;
18 my $sn = $INC{'Sub/Name.pm'} && eval { Sub::Name->VERSION(0.08) };
20 $su = eval { require Sub::Util; } && defined &Sub::Util::set_subname;
22 $sn = eval { require Sub::Name; Sub::Name->VERSION(0.08) };
26 *_subname = $su ? \&Sub::Util::set_subname
27 : $sn ? \&Sub::Name::subname
29 *_HAS_SUBNAME = ($su || $sn) ? sub(){1} : sub(){0};
34 # Need to prototype as @ not $$ because of the way Perl evaluates the prototype.
35 # Keeping it at $$ means you only ever get 1 sub because we need to eval in a list
36 # context & not a scalar one
39 my ( $try, @code_refs ) = @_;
41 # we need to save this here, the eval block will be in scalar context due
43 my $wantarray = wantarray;
45 # work around perl bug by explicitly initializing these, due to the likelyhood
46 # this will be used in global destruction (perl rt#119311)
47 my ( $catch, @finally ) = ();
49 # find labeled blocks in the argument list.
50 # catch and finally tag the blocks by blessing a scalar reference to them.
51 foreach my $code_ref (@code_refs) {
53 if ( ref($code_ref) eq 'Try::Tiny::Catch' ) {
54 croak 'A try() may not be followed by multiple catch() blocks'
56 $catch = ${$code_ref};
57 } elsif ( ref($code_ref) eq 'Try::Tiny::Finally' ) {
58 push @finally, ${$code_ref};
61 'try() encountered an unexpected argument ('
62 . ( defined $code_ref ? $code_ref : 'undef' )
63 . ') - perhaps a missing semi-colon before or'
68 # FIXME consider using local $SIG{__DIE__} to accumulate all errors. It's
69 # not perfect, but we could provide a list of additional errors for
72 # name the blocks if we have Sub::Name installed
73 _subname(caller.'::try {...} ' => $try)
76 # set up scope guards to invoke the finally blocks at the end.
77 # this should really be a function scope lexical variable instead of
78 # file scope + local but that causes issues with perls < 5.20 due to
80 local $_finally_guards{guards} = [
81 map { Try::Tiny::ScopeGuard->_new($_) }
85 # save the value of $@ so we can set $@ back to it in the beginning of the eval
86 # and restore $@ after the eval finishes
91 # failed will be true if the eval dies, because 1 will not be returned
93 my $failed = not eval {
96 # evaluate the try block in the correct context
99 } elsif ( defined $wantarray ) {
105 return 1; # properly set $failed to false
108 # preserve the current error and reset the original value of $@
112 # at this point $failed contains a true value if the eval died, even if some
113 # destructor overwrote $@ as the eval was unwinding.
115 # pass $error to the finally blocks
116 push @$_, $error for @{$_finally_guards{guards}};
118 # if we got an error, invoke the catch block.
120 # This works like given($error), but is backwards compatible and
121 # sets $_ in the dynamic scope for the body of C<$catch>
123 return $catch->($error);
126 # in case when() was used without an explicit return, the C<for>
127 # loop will be aborted and there's no useful return value
132 # no failure, $@ is back to what it was, everything is fine
133 return $wantarray ? @ret : $ret[0];
138 my ( $block, @rest ) = @_;
140 croak 'Useless bare catch()' unless wantarray;
142 _subname(caller.'::catch {...} ' => $block)
145 bless(\$block, 'Try::Tiny::Catch'),
151 my ( $block, @rest ) = @_;
153 croak 'Useless bare finally()' unless wantarray;
155 _subname(caller.'::finally {...} ' => $block)
158 bless(\$block, 'Try::Tiny::Finally'),
164 package # hide from PAUSE
165 Try::Tiny::ScopeGuard;
167 use constant UNSTABLE_DOLLARAT => ("$]" < '5.013002') ? 1 : 0;
175 my ($code, @args) = @{ $_[0] };
177 local $@ if UNSTABLE_DOLLARAT;
183 "Execution of finally() block $code resulted in an exception, which "
184 . '*CAN NOT BE PROPAGATED* due to fundamental limitations of Perl. '
185 . 'Your program will continue as if this event never took place. '
186 . "Original exception text follows:\n\n"
187 . (defined $@ ? $@ : '$@ left undefined...')
202 You can use Try::Tiny's C<try> and C<catch> to expect and handle exceptional
203 conditions, avoiding quirks in Perl and common mistakes:
205 # handle errors with a catch handler
209 warn "caught error: $_"; # not $@
212 You can also use it like a standalone C<eval> to catch and ignore any error
213 conditions. Obviously, this is an extreme measure not to be undertaken
216 # just silence errors
223 This module provides bare bones C<try>/C<catch>/C<finally> statements that are designed to
224 minimize common mistakes with eval blocks, and NOTHING else.
226 This is unlike L<TryCatch> which provides a nice syntax and avoids adding
227 another call stack layer, and supports calling C<return> from the C<try> block to
228 return from the parent subroutine. These extra features come at a cost of a few
229 dependencies, namely L<Devel::Declare> and L<Scope::Upper> which are
230 occasionally problematic, and the additional catch filtering uses L<Moose>
231 type constraints which may not be desirable either.
233 The main focus of this module is to provide simple and reliable error handling
234 for those having a hard time installing L<TryCatch>, but who still want to
235 write correct C<eval> blocks without 5 lines of boilerplate each time.
237 It's designed to work as correctly as possible in light of the various
238 pathological edge cases (see L</BACKGROUND>) and to be compatible with any style
239 of error values (simple strings, references, objects, overloaded objects, etc).
241 If the C<try> block dies, it returns the value of the last statement executed in
242 the C<catch> block, if there is one. Otherwise, it returns C<undef> in scalar
243 context or the empty list in list context. The following examples all
244 assign C<"bar"> to C<$x>:
246 my $x = try { die "foo" } catch { "bar" };
247 my $x = try { die "foo" } || "bar";
248 my $x = (try { die "foo" }) // "bar";
250 my $x = eval { die "foo" } || "bar";
252 You can add C<finally> blocks, yielding the following:
255 try { die 'foo' } finally { $x = 'bar' };
256 try { die 'foo' } catch { warn "Got a die: $_" } finally { $x = 'bar' };
258 C<finally> blocks are always executed making them suitable for cleanup code
259 which cannot be handled using local. You can add as many C<finally> blocks to a
260 given C<try> block as you like.
262 Note that adding a C<finally> block without a preceding C<catch> block
263 suppresses any errors. This behaviour is consistent with using a standalone
264 C<eval>, but it is not consistent with C<try>/C<finally> patterns found in
265 other programming languages, such as Java, Python, Javascript or C#. If you
266 learnt the C<try>/C<finally> pattern from one of these languages, watch out for
271 All functions are exported by default using L<Exporter>.
273 If you need to rename the C<try>, C<catch> or C<finally> keyword consider using
274 L<Sub::Import> to get L<Sub::Exporter>'s flexibility.
280 Takes one mandatory C<try> subroutine, an optional C<catch> subroutine and C<finally>
283 The mandatory subroutine is evaluated in the context of an C<eval> block.
285 If no error occurred the value from the first block is returned, preserving
288 If there was an error and the second subroutine was given it will be invoked
289 with the error in C<$_> (localized) and as that block's first and only
292 C<$@> does B<not> contain the error. Inside the C<catch> block it has the same
293 value it had before the C<try> block was executed.
295 Note that the error may be false, but if that happens the C<catch> block will
298 Once all execution is finished then the C<finally> block, if given, will execute.
302 Intended to be used in the second argument position of C<try>.
304 Returns a reference to the subroutine it was given but blessed as
305 C<Try::Tiny::Catch> which allows try to decode correctly what to do
306 with this code reference.
310 Inside the C<catch> block the caught error is stored in C<$_>, while previous
311 value of C<$@> is still available for use. This value may or may not be
312 meaningful depending on what happened before the C<try>, but it might be a good
313 idea to preserve it in an error stack.
315 For code that captures C<$@> when throwing new errors (i.e.
316 L<Class::Throwable>), you'll need to do:
337 Intended to be the second or third element of C<try>. C<finally> blocks are always
338 executed in the event of a successful C<try> or if C<catch> is run. This allows
339 you to locate cleanup code which cannot be done via C<local()> e.g. closing a file
342 When invoked, the C<finally> block is passed the error that was caught. If no
343 error was caught, it is passed nothing. (Note that the C<finally> block does not
344 localize C<$_> with the error, since unlike in a C<catch> block, there is no way
345 to know if C<$_ == undef> implies that there were no errors.) In other words,
346 the following code does just what you would expect:
351 # ...code run in case of error
354 print "The try block died with: @_\n";
356 print "The try block ran without error.\n";
360 B<You must always do your own error handling in the C<finally> block>. C<Try::Tiny> will
361 not do anything about handling possible errors coming from code located in these
364 Furthermore B<exceptions in C<finally> blocks are not trappable and are unable
365 to influence the execution of your program>. This is due to limitation of
366 C<DESTROY>-based scope guards, which C<finally> is implemented on top of. This
367 may change in a future version of Try::Tiny.
369 In the same way C<catch()> blesses the code reference this subroutine does the same
370 except it bless them as C<Try::Tiny::Finally>.
376 There are a number of issues with C<eval>.
380 When you run an C<eval> block and it succeeds, C<$@> will be cleared, potentially
381 clobbering an error that is currently being caught.
383 This causes action at a distance, clearing previous errors your caller may have
386 C<$@> must be properly localized before invoking C<eval> in order to avoid this
389 More specifically, C<$@> is clobbered at the beginning of the C<eval>, which
390 also makes it impossible to capture the previous error before you die (for
391 instance when making exception objects with error stacks).
393 For this reason C<try> will actually set C<$@> to its previous value (the one
394 available before entering the C<try> block) in the beginning of the C<eval>
397 =head2 Localizing $@ silently masks errors
399 Inside an C<eval> block, C<die> behaves sort of like:
403 return_undef_from_eval();
406 This means that if you were polite and localized C<$@> you can't die in that
407 scope, or your error will be discarded (printing "Something's wrong" instead).
409 The workaround is very ugly:
420 =head2 $@ might not be a true value
428 because due to the previous caveats it may have been unset.
430 C<$@> could also be an overloaded error object that evaluates to false, but
431 that's asking for trouble anyway.
433 The classic failure mode is:
435 sub Object::DESTROY {
440 my $obj = Object->new;
449 In this case since C<Object::DESTROY> is not localizing C<$@> but still uses
450 C<eval>, it will set C<$@> to C<"">.
452 The destructor is called when the stack is unwound, after C<die> sets C<$@> to
453 C<"foo at Foo.pm line 42\n">, so by the time C<if ( $@ )> is evaluated it has
454 been cleared by C<eval> in the destructor.
456 The workaround for this is even uglier than the previous ones. Even though we
457 can't save the value of C<$@> from code that doesn't localize, we can at least
458 be sure the C<eval> was aborted due to an error:
460 my $failed = not eval {
466 This is because an C<eval> that caught a C<die> will always return a false
471 Using Perl 5.10 you can use L<perlsyn/"Switch statements">.
473 =for stopwords topicalizer
475 The C<catch> block is invoked in a topicalizer context (like a C<given> block),
476 but note that you can't return a useful value from C<catch> using the C<when>
477 blocks without an explicit C<return>.
479 This is somewhat similar to Perl 6's C<CATCH> blocks. You can use it to
480 concisely match errors:
485 when (/^Can't locate .*?\.pm in \@INC/) { } # ignore
495 C<@_> is not available within the C<try> block, so you need to copy your
496 argument list. In case you want to work with argument values directly via C<@_>
497 aliasing (i.e. allow C<$_[1] = "foo">), you need to pass C<@_> by reference:
500 my ( $self, @args ) = @_;
501 try { $self->bar(@args) }
509 try { $_ = $self->bar($_) for @$args }
514 C<return> returns from the C<try> block, not from the parent sub (note that
515 this is also how C<eval> works, but not how L<TryCatch> works):
525 say "this text WILL be displayed, even though an exception is thrown";
528 Instead, you should capture the return value:
535 return unless $success;
537 say "This text WILL NEVER appear!";
540 sub parent_sub_with_catch {
546 # do something with $_
547 return undef; #see note
549 return unless $success;
551 say "This text WILL NEVER appear!";
554 Note that if you have a C<catch> block, it must return C<undef> for this to work,
555 since if a C<catch> block exists, its return value is returned in place of C<undef>
556 when an exception is thrown.
560 C<try> introduces another caller stack frame. L<Sub::Uplevel> is not used. L<Carp>
561 will not report this when using full stack traces, though, because
562 C<%Carp::Internal> is used. This lack of magic is considered a feature.
564 =for stopwords unhygienically
568 The value of C<$_> in the C<catch> block is not guaranteed to be the value of
569 the exception thrown (C<$@>) in the C<try> block. There is no safe way to
570 ensure this, since C<eval> may be used unhygienically in destructors. The only
571 guarantee is that the C<catch> will be called if an exception is thrown.
575 The return value of the C<catch> block is not ignored, so if testing the result
576 of the expression for truth on success, be sure to return a false value from
584 return; # avoid returning a true value;
591 C<$SIG{__DIE__}> is still in effect.
593 Though it can be argued that C<$SIG{__DIE__}> should be disabled inside of
594 C<eval> blocks, since it isn't people have grown to rely on it. Therefore in
595 the interests of compatibility, C<try> does not disable C<$SIG{__DIE__}> for
596 the scope of the error throwing code.
600 Lexical C<$_> may override the one set by C<catch>.
602 For example Perl 5.10's C<given> form uses a lexical C<$_>, creating some
610 warn $_; # will print $foo, not the error
611 warn $_[0]; # instead, get the error like this
616 Note that this behavior was changed once again in L<Perl5 version 18
617 |https://metacpan.org/module/perldelta#given-now-aliases-the-global-_>.
618 However, since the entirety of lexical C<$_> is now L<considered experimental
619 |https://metacpan.org/module/perldelta#Lexical-_-is-now-experimental>, it
620 is unclear whether the new version 18 behavior is final.
630 Much more feature complete, more convenient semantics, but at the cost of
631 implementation complexity.
635 Automatic error throwing for builtin functions and more. Also designed to
636 work well with C<given>/C<when>.
640 A lightweight role for rolling your own exception classes.
644 Exception object implementation with a C<try> statement. Does not localize
647 =item L<Exception::Class::TryCatch>
649 Provides a C<catch> statement, but properly calling C<eval> is your
652 The C<try> keyword pushes C<$@> onto an error stack, avoiding some of the
653 issues with C<$@>, but you still need to localize to prevent clobbering.
657 =head1 LIGHTNING TALK
659 I gave a lightning talk about this module, you can see the slides (Firefox
662 L<http://web.archive.org/web/20100628040134/http://nothingmuch.woobling.org/talks/takahashi.xul>
666 L<http://web.archive.org/web/20100305133605/http://nothingmuch.woobling.org/talks/yapc_asia_2009/try_tiny.yml>